The black sooting is indicitive of all fossil fuel burning. It is the lack of complete combustion and lack of proper fuel/air mixture. Sooting usually comes from not enough air or too much fuel. In the matter of return air. Is the basement part of the condition area that the furnace provides heat to, if so your return air duct could be the condition area it self. In other words its pulling the return through the house and down through the doorways. That being the case, it would do well to leave the basement door open at all times and I would hope that there is a supply going to the basement. You might have a combustion analysis done on the system.